The most common and most likely answer to this crossword clue is the 5 letter word SAFES
I've seen this crossword clue in The Times
S
A
F
E
S
More clues leading to the result SAFES
Buddy explains!

The straight part
We know that "Strongboxes" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case strongboxes = safes.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does SAFES mean?
SAFES noun- A box, usually made of metal, in which valuables can be locked for safekeeping.
- A condom.
- A ventilated or refrigerated chest or closet for securing provisions from noxious animals or insects.
- A safety bicycle.
- To make something safe.